The present invention relates to the exposure of photosensitive materials in reprographics. It is a question here of the exposure especially of films and printing plates, the exposure devices having one or more metal-halide lamps as a light source.
For metal-halide lamps to reach the desired light flux spectrum, the discharge container must be at a definite minimum temperature. The first thing that happens when this temperature is undershot is that the metal halides introduced to enrich definite spectral lines condense, so that with little change in the operating power consumption the emission spectrum of the lamp falls back to that of an undoped mercury vapor lamp. As the temperature sinks further, mercury also starts to condense increasingly, so that the internal pressure, and therefore the operating power consumption of the lamp drops.
In the graphics industry, the exposure, especially that of the UV-sensitive films and printing plates, requires emission of radiation be as constant as possible. This results in the requirement for a constant lamp temperature, which would mean the lamp burning continually at full illuminating power. This would be unfavorable, because exposures last for only relatively short periods between which interruptions occur in which the light flux is not required. This arises because before each exposure the material to be exposed (master and photosensitive material) must be positioned, and is normally, in addition, brought into intimate contact by the application of a vacuum. The time intervals required for this purpose amount typically to two minutes.
Admittedly lamps have been developed which are completely switched off in the pauses between the exposures, and cold started again in each case, for each exposure. However, this is unsatisfactory because of the necessary starting times in the order of 30 seconds.
Since, therefore, it is not possible in most applications to turn off the lamp completely between the exposures, and disadvantageous to allow continual burning at full power because of the energy consumption necessary, of the required cooling and of the effect on the operating life, it is usual to switch the exposure devices over to a power reduced stand-by operation ("stand-by power") in the pauses between the exposures.
In the known exposure procedures a specific stand-by power is provided, which makes available almost the entire light flux of the required spectrum virtually without a delay at switch-over to the full illuminating power. These stand-by powers normally lie in the range of 35% to 60% of the full illuminating power (electric power).